For the purpose of a reduction in fuel consumption, much research has been now carried out to reduce the engine displacement and the number of cylinders, and its results are turned into practical use. Yet, no engine (internal combustion engine) with a smaller number of cylinders has come into reality because of a problem of maximum torque. Generally speaking, rolling vibrations caused by fluctuations in torque is a problem for reciprocating engines with a small number of cylinders, particularly reciprocating engines with three or less cylinders.
As a countermeasure for this problem, there is a device provided with an inertial system which rotates in a direction reverse to that of the engine, and configured to reduce rolling vibrations of the engine by offsetting torque recoil force caused around a crankshaft by torque recoil force generated by the inertial system (see Patent Document 1, for example). This device is so-called a Heron balancer. Devices of this type configured to create the inertial system that rotates in the direction reverse to that of the engine include: a device which uses a generator; a device to which a weight is newly added; and a device in which a weight is added to a primary balancer.
In addition, there is also a device including two balancer shafts disposed as rotary shafts in parallel with a rotational axis of a crankshaft, and a generator-driver installed on at least one of the two balancers, and configured to offset fluctuations in torque by braking torque and driving torque of the generator-driver (see Patent Document 2, for example).
However, another problem arises from the above-mentioned devices because the inertial system, which rotates in the direction reverse to that of the engine, is gear-driven. The engine produces torque recoil force accompanying the fluctuations in the torque because of its intermittent combustion. Particularly, as the number of cylinders becomes smaller, the value representing the torque recoil force becomes larger. As a result, the torque recoil force accompanying the fluctuations in the torque causes a rotational fluctuation speed. At the time of acceleration, the gears are rotated in such a way that a tooth surface on one side presses a tooth surface on the driven side. At the time of deceleration, however, since the gear on the driven side belongs to a different inertial system, the tooth surfaces are detached from each other, and the gear on the driven side comes into contact with a back surface of the next gear tooth. At this time, a clanking noise occurs. Subsequently, in the next acceleration, the original tooth surfaces come into contact again to drive the gear. At this time, a clanking noise occurs again. In short, in exchange for the reduction in the rolling vibrations, each of the above-mentioned devices gear-driven by the crankshaft causes gear noises such as the clanking noises.
The phenomenon becomes more noticeable as the moment of inertia on the driven side is increased for the purpose of offsetting the torque recoil force. The distance each tooth moves is called gear backlash, which cannot be reduced to zero. The clanking noises become louder as the moment of inertia on the driven side becomes larger.
To deal with this problem, there is a device including two balancer shafts driven by gears, in which an oil pump is connected to one of the two balancer shafts. The device is configured to reduce gear noises by making the phase of the rotational fluctuation speed of the engine coincide with the phase of the rotational fluctuation speed of the oil pump (see Patent Document 3, for example).
This device, however, produces loud clanking noises from torsional vibrations of the shaft caused by the torque recoil force associated with the fluctuations in the torque, because the oil pump functioning as a large inertial body is directly attached via the gear. Furthermore, this device requires large modifications to be made to the engine.
In sum, when the rotational body gear-driven by the crankshaft and configured to rotate in the direction reverse to that of the crankshaft is provided for the purpose of suppressing the torque recoil force associated with the fluctuations in the torque, the moment of inertia becomes larger and leads to occurrence of gear noises. Furthermore, the larger moment of inertia causes problems such as deterioration in fuel efficiency and delay in acceleration response.